Gas condensates arise from the destructive distillation of organic material such as coal, e.g. in coal gasification plants, in coke furnaces and others and in carbonization processes, these condensates containing various impurities which are detrimental to the environment and require removal. After removal of the noxious or toxic impurities, at least some of which are recoverable for economical use, the balance of the condensate may be discharged into an effluent stream from the plant. Stringent environmental protection requirements in recent years have made it necessary to subject the condensate before discharge as the effluent to biological cleaning, e.g. an activated sludge process whereby microorganisms and oxygen cause decomposition of residual organic compounds to produce a decantate which is practically clean and can be discharged into streams or the like without the danger of contaminating them.
Biological purification may take place in activated sludge tanks, sedimenting and settling tanks with or without aid of flocculants and with or without injection of oxygen to promote decomposition.
In general the degree of contamination of the condensate to be subjected to biological cleaning can be measured by its biological oxygen demand (BOD). It is desirable to subject to purification a gas condensate having as low a biological oxygen demand as possible so that the residence time required for biological cleaning is minimized and the product of the biological cleaning is as pure and free from contaminants as possible.
The gas/water-vapor mixture derived from a carbonization or coking process can be cooled to form the gas condensate which contains ammonia, acid gases such as sulfur dioxide (SO.sub.2), carbon dioxide (CO.sub.2) and hydrogen sulfide (H.sub.2 S), tars, neutral oils, phenols and tar bases. Tar bases are tarry substances or substances trapped in or entrained by the tars which have a basic character and include pyridine-containing tars. The term "acid gases" is used to describe gases with an acidic character such as the aforementioned SO.sub.2, CO.sub.2 and H.sub.2 S. The bases contained in the condensate include ammonia (NH.sub.3) and the aforementioned tar bases.
In practice it is found that the phenol and tar bases content is dependent on the process temperatures, at 600.degree. C., 20 to 30 grams per liter of phenols and about 5 grams per liter of tar bases are present in the condensate.
It has been proposed heretofore to treat such gas condensates by an initial mechanical clarification resulting in the separation of tars and its filtration from the liquid phase, to treat the liquid phase with a solvent for the phenols (extractive dephenolization), to steam distill off components susceptible to steam distillation such as ammonia, hydrogen sulfide, carbon dioxide and HCN, and thereafter to subject the liquid phase to biological purification.
The effluent from the final step in the process can generally be discharged into the environment with no detrimental effect and usually meets the stringent requirements for discharge effluents and waste water regulations.
It has already been found that extractive dephenolization considerably reduces the biological oxygen demand of the water (see H. J. WURM, "Untersuchungen uber die Wirtschaftlichkeit der wichtigsten physikalisch-chemischen Verfahren zur Entphenolung von Kokereiabwassern", Dissertation, Technische Hochschule Aachen, Germany (1973)--("Investigations into the economies of the significant physical-chemical process in the dephenolization of coking waste water"). In this operation, the removal of steam distillable phenols is up to 99.9% while other phenols and tar bases are removed only in minor proportion. The remaining tar bases, phenolic matter and other substances which cannot be removed by the prior art processes appear to contribute substantially to the biological oxygen demand of the condensate after the process steps which prepare the condensate for biological decontamination.